1. Field of the Invention
The invention relates to a spectrometer for surface analysis of a sample, the spectrometer having optical sample viewing capability, and to a corresponding method for surface analysis spectroscopy. The invention relates particularly, but not exclusively, to sample analysis by means of secondary electron energy spectroscopy, such as X-ray photoelectron spectroscopy or Auger electron spectroscopy.
2. Description of the Related Art
Chemical and physical analysis of the surface of a sample generally requires the excitation of that surface with a primary beam of “particles”, such as ions, electrons, photons or atoms, and the detection of secondary particles which are emitted from the surface and whose characteristic energy or mass is measured. The energy or mass spectrum obtained is used to provide information about the chemical or elemental composition of the sample. There are many known techniques employing this form of analysis, including for example Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS), and secondary ion mass spectrometry (SIMS).
The majority of these techniques use a vacuum system in which the analysis is performed. A primary particle beam generator is employed to provide a beam of irradiation which is directed towards an analysis area of the spectrometer, at which location is placed a sample, held in a vacuum analysis chamber. The secondary charged particles emitted from the surface of the sample are then collected by a charged particle collection lens arrangement from where they pass into a spectroscopic analyser in which their energies or masses are determined.
In addition to making spectral observations of the secondary particles from the sample, it is generally desirable also to be able to view the surface of the sample visually, using an optical microscope. Such a microscope may be arranged so that the surface can be visually observed while secondary particles are collected from the surface. Alternatively, or additionally, the microscope may be used to identify and define an area of analysis, especially where analysis is to be made of a small area of the sample surface. The ability to perform simultaneous visual and spectral observations is particularly desirable during system set-up.
For more than 20 years, surface analysis systems have been manufactured with the charged particle collection lens arrangement being mounted at, or close to, the normal to the surface of a sample. Such systems include the Thermo VG ESCALAB systems, manufactured by Thermo Electron Corporation of Delaware, USA, and KRATOS ULTRA XPS systems, manufactured by KRATOS Analytical (a subsidiary of SHIMADZU Corporation), of Manchester, United Kingdom, among others. Where a system includes an optical microscope, this is mounted at an angle to the normal to the sample surface, typically 45°. A separate light source providing illumination to the area under investigation on the sample surface is then similarly mounted, at any angle to the axis of the optical microscope.
However, the quality of the viewed image from this sample viewing geometry is generally poor, with the sample image only being truly in focus at the centre of the field of view and out of focus towards the edges, because of the angle from which the sample is viewed. In addition, when the sample surface is illuminated by the light source, shadows formed as a result of the sample's surface topography can degrade the image quality, for the same reason. Furthermore, when the sample height is changed, the analysis position on the sample, as viewed by the optical microscope, also appears to move. This sample viewing geometry is therefore not desirable for identifying the analysis position.
An arrangement which provides optical viewing normal to the sample is the electron probe micro-analyser (EPMA) manufactured by Cameca of Courbevoie Cedex, France. However, the EPMA is a wavelength-dispersive X-ray microanalysis (WDX) system, in which the secondary particles are not charged particles, but X-ray photons. The optical axis leaving the sample for the microscope is mounted normal to the sample surface, along with the primary electron beam generator. The primary beam's probe-forming optics produce a very narrow beam, which is arranged to pass through a small hole in the centre of a conventional microscope Schwarzschild reflective objective disposed just above the sample. The secondary particles, which are X-ray photons, are collected for measurement at approximately 45° to the normal.
For the Cameca EPMA, the use of off-axis X-ray collection leads to a reduced sensitivity, particularly for soft X-ray emission. Moreover, this system cannot be used for secondary charged particle analysis.
Secondary charged particle systems having the optical microscope mounted normal to the sample surface are also known; for example, the SSI X and S probe systems, manufactured by Surface Science Instruments of California, USA, and the Thermo Thetaprobe, manufactured by Thermo Electron Corporation. In these systems, the charged particle collection lens arrangement is mounted at an angle of typically 55° to the normal.
While visual observations with such sample viewing geometries may be of better quality, this is at the expense of potentially significant degradation in the analysis sensitivity of the instrument for the SSI X and S probe and Thermo Thetaprobe systems, due to the off-axis collection of the secondary charged particles.
Generally, in order to save space in the laboratory, several different surface analysis instruments are provided together, in a multitechnique system. Such a system may then be operated in any particular surface analysis mode, or sequence of modes on the same sample, as required by the application.
Each different analysis mode may require its own primary column and secondary column. Including access for an optical camera, a sample illumination source, a vacuum pump, sample entry and the like, the analysis chamber for such a multitechnique system must therefore be provided with a large number of chamber ports. Furthermore, all of the ports which are relevant for the collection of surface analysis data must be directed at the sample analysis area.
The actual location and configuration of each chamber port is accordingly a matter of balancing the competing desires of accommodating the hardware for each instrument without overcrowding the chamber; favourably positioning the primary beam column for each instrument; favourably positioning the secondary beam column for each instrument; and favourably positioning the ports used in common by all of the analysis instruments.
The ESCALAB system is one example of a mutitechnique system, a typical chamber arrangement for which is illustrated in FIG. 8. Table 1 below lists the port assignments for the arrangement.
TABLE 1PortPortassignmentAssignment description801Lens assemblyCollection lens for charged secondary particles.802Electron gunSource of primary electron beam for AUGER experiments803X-rayPrimary X-ray source using a quartz crystal X-raymonochromatormonochromator804X-ray sourcePrimary un-monochromated X-ray source805Entry chamberSample pump-down chamber or sample preparation chamber806Quad SIMSQuadrupole detector for SIMS experiments807ScintillatorLow energy electron detector for scanning electronmicroscope (SEM) imaging808Ion gunIon beam source for depth profiling/sample cleaning ionscattering spectroscopy (ISS) or SIMS809View portLarge glass window to view sample by eye810CCD cameraGlass window through which CCD microscope sample viewingoccurs811Stage portMounting for stage to move sample around inside vacuum812LightGlass window for sample illumination813Iris controlMounting for collection lens F/number aperture mechanism814SpareOften used for residual gas analysis (RGA) or anadditional light source815Flood gunLow energy electron gun for charge neutralisation ofsamplenotPumping portPort through which chamber may be evacuated to ultra highshownvacuumnotBeam portPort for connecting system to beam line primary X-rayshownsources820Magnetic lensMounting for magnetic collection lens (fitted belowsample)821Gas dosingPort for admission of gas for surface reactionexperiments822U.V. sourceSource of ultraviolet primary radiation for UPS analysis
In view of the formidable problem of optimising the configuration of the instrument ports in an analysis chamber, a number of alternative approaches have been proposed.
One approach, employed in the NanoSIMS system, also manufactured by Cameca, involves performing the spectral observation and visual observation of the sample at different locations. In the NanoSIMS system, the sample is translated between an analysis position for spectral observation and a remote position for optical viewing.
Another approach, used in the Quantum and Quantera systems, manufactured by Ulvac-PHI of Kanagawa, Japan, the analysis chamber is entirely separate from the optical microscope, and CCD images are recorded on the microscope workbench before the sample is introduced into the analysis chamber.
While both visual and spectral observations may be made with the optical microscope axis and the collection lens arrangement axis respectively being normal to the sample surface, this approach has a significant disadvantage in that the sample cannot be optically viewed during spectral analysis. In addition, the co-ordinate system for mounting and moving the sample must be very accurate.
It would be desirable, therefore, to provide a charged particle spectrometer capable of making spectral observations of secondary charged particles with high sensitivity and of producing optical images of the sample with high quality. The invention aims to address the foregoing desires by providing an improved charged particle spectrometer.